There is nothing that will make you feel like an old MMO grouch more than hearing people get excited about the prospect of reusing content you saw enough of the first time around.
Naxxramas! Onyxia! Heroic Deadmines and Shadowfang! All the monster models that get recoloured, renamed, and reused! Scaling instances in LOTRO!
I don’t hate any of these things. There is a balance between nostalgia (Hey! I remember back in the day when we had to walk uphill backwards to get the key to blahblahblah) and burnout (if I ever have to see that fight again I’m going to rip someone’s arms out). But modern MMOs can’t afford to leave too much old content hanging around unused – it just isn’t cost effective.
Also, the new players hear the older ones talking wistfully about how the game was back in the day. They don’t see the annoying parts that get skipped in reminiscences, they just feel that they missed out and are being left out of the conversation.
I wrote once that after about 3-4 years, an online game would have to change or else it would die as the first wave of players moved on and the newer ones could never catch up. I now think that after 3-4 years, older players will be nudged to move on because smart devs have realised this fact and will then start to direct most of the content to the newer players.
So if your favourite game starts to make you feel old, grouchy, out of touch, and as if any new elements are no longer aimed at you … maybe you’re right and it’s time to move on.
Welcome to Spinksville! 
The endgame is always limited – to the latest batch of dungeons/raids.
There is also no point in having too much choice. Not even advanced cross-server systems or one-server worlds can muster enough people to fill all the many dungeons.
The only thing that can serve a huge number of dungeons is really a system that randomly send people to this or that dungeon like the dungeon finder.
Imagine I give people 1000 perfectly fun and viable dungeons for choice, the community would have to find gentlemen agreements which few of that they actually want to play and focus on.
Right now we have a quest based system of MMO narrative. It is static and linear and stacking new stuff on top of old stuff.
-> Why not try a system with an ONGOING narrative. Yes, old content would be gone. Vashj is dead, and now Arthas is dead.
In fact all Northrend quests would be gone, outdated. The campaign is over.
A system without everlasting levelling and gear related power creep could offer an ongoing world with weekly events for the whole population and would also do away with what people do not seem to appreciate anymore at all, levelling through the “old” content they have to wade through before getting to the real game, the endgame.
Sure, the end game is limited. But you can see why people wonder at seeing good content taken out of the game just because the current batch of endgame players have played it to death. Especially if there’s any way to make it scale better in future.
I think it’s as much to do with the punctuated nature of levelling in these games. You level to the cap, spend a year or so doing a variety of endgame stuff … then an expansion comes out and you do the same thing again. That’s why the content doesn’t fit a newbie levelling curve so well.
I see the other side of this in LOTRO since my raid group is tackling old content (which is new and fun to me) and I’m glad that it is there as an option. It makes no sense to say ‘oops, this dungeon disappeared suddenly’
Speaking as an inveterate leveler, I’ve yet to get bored of old content. Of course I never raided Vanilla or BC, so a lot of that stuff would be new to me anyway, and I can kind of see how I’d hate to run Northrend instances once cataclysm is out because, dude, I’ve done those things a hundred times each.
I *do* dislike the recycling of old content at max level, because it feels too much like a deliberate nostalgia-tripping. There was no *point* to Ony-80, it had nothing to do with Wrath at all (ironically it would have made significantly more sense to update the old girl for Cataclysm). I far prefer what they seem to be doing with the old content in Cataclysm, which is retuning it to be accessible again for people who are leveling. After all, that was kind of what it was there for in the first place.